Code Geass is a difficult series to review, simply because all of its defining features are things which some consider strengths and others weaknesses. Perhaps this is why there's such a rift in the fanbase, and it's one of those series which everyone seems to either love or hate. SO, I'm going to step out of my usual reviewing template and go over what exactly Code Geass is, and you can decide for yourself whether these are positive qualities or negative ones.
Code Geass is over the top. The characters are over the top, the setting is over the top, the dialogue is over the top, the performances are over the top, the music is over the top, and the battles are over the top... everything is over the top. If it's not over the top at any given moment, something's wrong--you've probably hit your pause button by accident or something. There's no such thing as a non-dynamic entry in this series, or a conflict that doesn't conclude with an epic twist turning the tables grossly in one side's favor in a wild display of military tact. Some might call this hammy. I call it thespian. And man oh man, I do love me some thespianism.
Code Geass is powerful... which is not entirely separate from its over-the-topness. Characters live (sometimes die), breathe, and interact, each with their own lives, goals, values, and motivations--and more than often, these wills clash. Everyone is out to change the world for the better, and each in their own unique way, leading the viewer to a good bit of philosophising. This also creates some damn powerful scenes liable to pull a tear or two unless you really find yourself detached from the characters. Some people might find this overbearing. I, myself, love really powerful and over-the-top series (like Death Note), and this more than qualifies.
Code Geass is moody. At times it wants to be a political drama series, and at times it wants to be a school comedy series, like jumping between Gundam and Ouran High School Host Club. I think this is what turns a lot of people off--some just don't like their peas and mashed potatoes touching, I suppose. The political drama aspects are much more prominent than the "school hijinx" aspects, which only occupy one or two scenes per episode with an occasional filler-esque episode thrown in (though, truthfully, there's no legitimate filler in Code Geass--good news for those who dislike filler), and the contrast makes sense in terms of the plot, given that the main character is the leader of a terrorist faction leading an undercover life as a student. Because of this, and because of the fact that I enjoy both drama series and school comedy series, I have no problem with this stark contrast. Others might.
Finally, Code Geass is pretty. This series is kind of the poster child of the "new wave" of animation, with its angular jaws, bright colors, pretty boy character designs, and shiny... well, everything. I'm a pretty hip old codger myself, but many seem to hold a very bitter resentment for this modern style of animation, so if it's something that really eats at you, you're probably not going to be able to sit through the entirety of Code Geass. If you're like me and don't really care one way or the other what kind of style a series employs as long as it looks good overall and is reasonably unique in its own right (which Geass is, with its exceptionally wiry and "idealized" character designs and well-constructed mecha frames), then this shouldn't be a problem. I think the designs add to the thespian style of the show, myself, looking at how medieval artwork often idealized the human form.
If you're a thespian with a love for emotional character interactions, an interest in all genres of anime, and a tolerance for different art styles, like me (or, if you enjoyed both Death Note and Gundam 00), then stop reading this and go watch this series right now--nowhere else will you find entries more dynamic, scripts more powerful, and plot twists less expected. If not? Well, maybe this show isn't for you, but it's worth giving a chance anyway. Code Geass is my personal favorite series of all time, with--in my opinion--the most powerful ending of all time, and while that's not until the second season, the first still ends on a pretty damn powerful note... albeit a cliffhanger. For that reason, I'd suggest waiting to start the series until you have both seasons available to you so you can go the whole distance. Once you do, though, you (hopefully) won't regret it.